I can't find anything on the phrase, but my guess is something either food related, e.g. meat grinding, or perhaps something to do with film projectors (although I suspect the phrase is older than film).

I wonder if it could have been derived from a woodworker assessing the results of his lathe? "Turning" is the process of making things on a lathe. Perhaps seeing how "it turns out" could refer to examining the results of one's lathe work?
–
user23037Jul 2 '12 at 5:01

Although I have no definitive answer here, I have an informed one. "Turn out" may come from the metaphor of "destiny or identity is a direction".

The idea of metaphors ruling our thinking and language is proposed by George Lakoff and Mark Turner, particularly in books like Metaphors We Live By and More Than Cool Reason. They present the idea that most figures of speech are based in a metaphorical understanding of the world, metaphors like "life is a journey" (so we use figures of speech such as, "We've come so far", "this is a milestone", "on the road to wealth", etc.).

I would think one metaphor is destiny and therefore identity is direction. Here, to "turn" means to literally change direction, like a compass pointer, and therefore metaphorically to change destiny or identity (of a person or a situation). So if a battle "turns", it changes its destiny. When someone "turns religious", they change their identity or destiny. When milk "turns", it changes its identity for the worse.

When we ask, "How did it turn out?" I think we are calling on this metaphor to say, "What was the identity or destiny of the situation at the end?". If things "turned out well" then they were good; etc. The "out" part may be an additional metaphor that says "out means revealed, in means concealed". So "turned out" means the direction/destiny was revealed.

Notice we also call on this identity or destiny is a direction when we say "how did things wind up?" (wind as in winding road, that is, direction).

A baker turns out a cake (or similar) onto a tray from the baking tin after baking in order to cool. The tin is turned and the cake comes out. This is critical moment - a point of assessment for the baker, and so the cake turns out well or badly. Whether this is the origin of the verb I don't know, but it's the most literal and least idiomatic use I can think of, and I'm not even a cook!

Removing something from a mold might have been the origin (or an origin, this being a common enough phrase to have more than one), but it's unlikely to have come from a baking context: historically, bread was most often baked without any sort of pan or form, i.e. directly on the floor of the oven.
–
MarthaªJul 2 '12 at 23:26

Etymonline does not mention exact phrase 'to turn out', but there is a short and simple entry for turnout

"audience," 1816

Macmillan lists the following, related meanings for the word turnout:

the number of people who come to an event

the number of voters in an election

Obviously, it would be hard to imagine that the word turnout was established before or independently of the phrase turn out.

However, it is possible to imagine that, similarly to nautical origin of 'turn in' - go to bed, there was a theatrical context in which the adverb out was added to the word. Once established as attendance or audience of a play, which is a result, the meaning could have been applied to any result of any event, even for small and personal events.

The above is a hypothesis, what follow is wild speculation:

Thinking about some translations of the phrases such as 'It turns out' to other languages, I came to a possible explanation why word out was chosen - the result of the show can be best measured by two things:

the mood of people leaving the theater

the number of people leaving the theater at the end of the show (and not before)

Both facts are established while people are going out of the theater - hence the word out and it seems compatible with history of meanings. Here's a fictional dialog - Q: "How was the audience last night?" A: "When they turned up they were many but when they turned out, there were few and looked bored."

Another explanation might be that the word was established during outdoor theater performances and hence the out.

Idiomatic phrases like "turn out" just cannot be taken literally, word for word. Over the course of time, the literal meaning has evolved into another layer of meaning that is no longer represented by the interpretation of the individual words in a verbatim phrase.

That is what idoms are: there is a difference between the word-for-word meaning and the meaning of the phrase as a whole. Open any idiomatic phrase dictionary for thousands more examples. As another example, "How are you doing?" has nothing to do with actually "doing" anything either. And if you were doing something, the grammatically correct question should be either "What are you doing?" or "How are you doing this?" But since it is an idiomatic phrase, it just means that somebody enquires after your well-being.

Each idiomatic phrase has its own history, and each will now mean more than the logical sum of their words.

Somewhere in the past, "turn out" probably had something to do with actually "turning" something, or things "going" somewhere, as icnivad has described so well in the previous post.

I can still sense an element of something changing from one state to another state in the phrase "turn out", but to me it is one of many set phrases, with a distinct meaning of its own that cannot be completely deciphered by interpreting the individual words of the phrase.

As a non-native speaker of English, I need to rely on a dictionary that tells me what "turn out" means, compared to the individual meanings of "turn" and "out".